1. Field of the Invention
The present invention relates to magnetic recording media having improved magnetic recording properties. In particular, the present invention relates to longitudinal (horizontal) recording media having an underlayer composed of a Cr-based alloy such as CrMo and an additive selected from B, Si and BN.
2. Description of Related Art
Thin film magnetic recording media are typically composed of multiple layers, including one or more magnetic recording layers, disposed on a rigid substrate. Traditionally, the magnetic layer includes magnetic grains that are grown vertically with the crystallographic c-axis (the easy magnetization axis) oriented longitudinally (i.e., in plane) with respect to the substrate.
The areal density of longitudinal magnetic recording media has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 Gbit/in2 have been demonstrated. However, further increases in areal densities are becoming difficult to achieve for a variety of reasons. One factor is the degradation of the signal-to-noise ratio (SNR) as the areal density increases. The SNR is a measure of the amplitude of the desired signal to the amplitude of the noise (i.e., undesired disturbances) and a high SNR is desirable. One way to increase the SNR is to decrease the noise. Low noise media can be achieved by reducing the magnetic grain size and/or narrowing the grain size distribution in the magnetic recording layer. Accordingly, it has become increasingly important to reduce the grain size and narrow the grain size distribution in order to further reduce media noise and increase the areal density of the media.
Due to the nature of epitaxial growth in thin film media, the properties of the magnetic layer can be controlled to some extent through proper selection of the layers underlying the magnetic layer. For example, it is known to use an underlayer disposed between the substrate and the magnetic layer in thin film recording media. The underlayer typically influences the crystalline structure, and hence the magnetic properties, of the overlying magnetic layer.
U.S. Pat. No. 4,652,499 by Howard discloses a magnetic recording medium including a magnetic layer and an underlayer. The magnetic layer is a CoPt or CoPtCr layer having a hexagonal-close packed structure (hcp) and the underlayer is a body-centered cubic (bcc) chromium-based alloy with a lattice cell constant greater than pure chromium, such as chromium-vanadium (CrV). The underlayer has a lattice cell constant that closely matches the lattice cell constant of the magnetic layer.
U.S. Pat. No. 5,900,324 by Moroishi et al. discloses a magnetic recording medium that includes two or more magnetic layers with one or more CrMo spacer layers between the magnetic layers. The medium also includes an underlayer in contact with a first magnetic layer, such as a CrMo alloy underlayer which may include other elements such as Zr, W, B, V, Nb, Ta, Fe, Ni, Re, Ce, Zn, P, Si, Ga, Hf, Al and Ti. It is disclosed that noise generation is suppressed by employing magnetic layers having a small thickness.
U.S. Pat. No. 6,287,429 by Moroishi et al. discloses a magnetic recording medium that includes a magnetic layer disposed on an intermediate layer. The intermediate layer includes chromium (Cr) and molybdenum (Mo) and the magnetic layer includes cobalt (Co) and platinum (Pt). It is disclosed that the intermediate layer may further include at least one element selected from the group consisting of tungsten (W), boron (B), vanadium (V), niobium (Nb), tantalum (Ta), iron (Fe), nickel (Ni), rhenium (Rh), copper (Cu), zirconium (Zr), zinc (Zn), phosphorus (P), silicon (Si), gallium (Ga), germanium (Ge), hafnium (Hf), aluminum (Al) and titanium (Ti). It is disclosed that the recording media has increased magnetic coercivity, leading to an increased areal density.
Despite the foregoing, there is a need for a magnetic recording medium including a magnetic recording layer having a reduced grain size and a narrow grain size distribution to enhance the magnetic properties of the recording layer, such as the areal density and the signal-to-noise ratio. It would also be advantageous if the magnetic recording layer had a high magnetic coercivity to further enhance the areal density.